7 RIG COMPONENTS
7.6 Circulating System
7.6.1 Drilling Fluids and Mud Conditioning Equipment
The drilling fluid is usually a liquid, but it can also be gas or air. If the drilling fluid is liquid, it is mostly made up of water, although, occasionally oil is a major component.
Both types of drilling fluids are called mud because that is what they appear to be.
Special clays, like bentonite, are added to give body to the mud. Barite and sometimes hematite is added to increase the density of the mud. Chemicals are used to control the viscosity of the mud and to improve the ability of the solid particles in the mud to deposit a layer on the wall of the hole. This cake is called a wall cake.
Drilling fluid returning from the borehole contains drilled cuttings, mud solids, other particles, and sometimes hydrocarbons - all of which must be removed before the mud is suitable for recirculation. In addition, treatment chemicals and clays must be added to the mud system from time to time to maintain the required properties. The equipment necessary to perform these functions:
• Mud Tanks or Mud Pits
• Mixing Hopper
• Standpipe and Rotary hoses
• Mud Conditioning Equipment o Shale Shakers
o Hydroclones (desanders, desilters) o Decanting Centrifuges
o Mud Cleaners o Vacuum Degassers 7.6.2 Mud Tanks and Pumps
The drilling fluid or mud is mixed and stored in the mud tanks or mud pits. A mud hopper is used to pour most of the dry ingredients needed to make the mud.
Agitators are used to keep mud stirred up in all parts of the mud pit. The most common types of agitators are the mud gun and the mechanical agitator.
A mud gun agitates the mud with a high velocity mud stream emanating from its nozzle. This nozzle could be directed to any portion of the mud tank. A mud gun requires a high pump pressure and a considerable volume of mud to get good results.
Even though several mud guns may be used, the agitating efficiency is low.
Mechanical agitators are more efficient than mud guns. They have paddle-like projections that are driven by a motor to mix the mud in the mud pits.
Figure 50 Pit room in an offshore rig.
The mud pump is the primary component of any circulating system. It is used to move large volumes of fluids of fluids at high pressures from the pits, through the drill stem, to the bit. There are at least two mud pumps in a rig.
There are two types of mud pumps:
Duplex – double-acting, reciprocating pumps; there are not many duplex pumps in operation today
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Figure 51 A duplex mud pump.
Triplex – single-acting pumps; have three pistons and are lighter and smaller than duplex pumps; can be operated at higher pressures than duplex pumps
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Figure 52 A triplex mud pump.
When air or gas is used, compressors replace mud pumps. Mud pits are not needed when air or gas is used to drill a well.
7.6.3 Mixing Hopper
Powdered chemicals and materials, as well as solids, needed by the mud are added using the mixing hopper. There are many types of hoppers in use.
through the jet and lowers the static pressure creating a vacuum causing the material in the hopper to be sucked and mixed with the mud.
Figure 53 A jet hopper.
7.6.4 Standpipe And Rotary Hose
The standpipe is a steel pipe mounted vertically on one side of the mast. The standpipe is firmly anchored to the derrick and it serves to suspend the rotary hose so that it is clear of the rig floor when the kelly/swivel or top drive system nears the rotary table. It allows the vertical movements of the drill string nearly twice the length of the rotary hose. It also allows drilling fluid to move from the mud lines coming out from the mud pump to the rotary hose.
Standpipes
Figure 54 Two standpipes on the drillfloor. Note: ILO pressure transducer on standpipe no. 1 (left).
The rotary hose or kelly hose is attached to a gooseneck on the standpipe and a gooseneck in the swivel.
7.6.5 Accessories
Drilled cuttings, other particles from the hole and sometimes gas, have to be removed from the mud system before it is suitable to be recirculated. Mud chemicals and water have to be regularly added to maintain the physical properties of the drilling fluid at desired levels. Shale shakers, the settling pits (or sand traps), desanders, desilters, centrifuges, degassers and mud cleaners remove unwanted solids and gas from the returning mud. The mixing hopper is used to add dry chemicals to the mud.
7.6.5.1 Shale Shaker
The shale shaker is the first mud cleaning equipment that encounters the returning mud. It contains one or more sloping and vibrating screens. The screens let the mud fall through but prevent large solids from going into the mud pits. The solids travel to the edge of the screen where they are “dumped”. Some of them are caught as geological samples.
There are two types of shale shakers:
• Circular/elliptical motion shale shakers
• Linear motion shale shakers
The circular/elliptical shale shaker uses elliptical rollers to generate a circular rocking motion to provide better solids removal through the screens.
The linear motion shale shaker a forward and back rocking motion to keep the fluids circulating through the screens.
Figure 55 A linear motion shaker.
7.6.5.2 Settling Pit or Sand Trap
The settling pit or sand trap is located below the shakers. It is the first pit that receives the returning mud from the shakers. The pit has a sloped bottom so that particles that passed through the shaker screens will segregate out, by gravity, and settle towards cleanout valves. These valves are opened periodically in order to dump the solids that have accumulated.
7.6.5.3 Centrifugal Separation Devices
There are two types of centrifugal separation devices namely:
Decanting centrifuges
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• Hydrocyclones 7.6.5.3.1 Decanting Centrifuges
coarse particles to the discharge port and back to the active system. The conveyor rotates in the same direction as the outer bowl but at a slightly slower speed.
Figure 56 Cross-section of a decanting centrifuge. Arrows indicate the path of solids and liquids (from EXLOG).
For low-density mud systems a single centrifuge unit set for total solids discard is used. The primary function of a centrifuge is not to control total percent solids in a system, but rather to maintain acceptable and desirable flow properties in that system.
Two centrifuges operating in series are recommended for the following systems:
Invert emulsion (i.e., synthetic and oil-based systems)
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High-density, water-based systems
Water-based systems in which base fluid is expensive (i.e., brines) Closed loop
Zero discharge
The first centrifuge unit is used to separate barite and return it to the mud system. The second unit processes the liquid overflow from the first unit, discarding all solids and returning the liquid portion to the mud system.
Note: Centrifuge efficiencies are influenced by mud weight and mud viscosity.
7.6.5.3.2 Hydrocyclones
Hydrocyclones, classified as desanders or desilters, are conical solids separation devices in which hydraulic energy is converted to centrifugal force in order to separate the solids from the rotating fluid. A pump is used to feed the drilling mud through a tangential opening into the feed chamber (the large end of the cone-shaped housing).
The centrifugal forces thus developed multiply the settling velocity of the heavier-phase material, forcing it toward the wall of the cone. The lighter particles move inward and upward in a spiraling vortex to the overflow opening at the top. The discharge at the top is the overflow or effluent. It is returned to the mud system. The discharge at the bottom that contains the coarse solids (which are discarded) is the underflow. The underflow should be in a fine spray with a slight suction at its center.
Figure 57 Hydroclone cross section. Arrows indicate direction of flow (from EXLOG).
Even though hydrocyclones are effective in removing solids from a drilling fluid, their use is not recommended for fluids that contain significant amounts of weighting materials or muds that have expensive fluid phases. When hydrocyclones are used with these fluids, not only will undesirable drilled solids be removed, but also the weight material along with base fluid, which can become cost-prohibitive.
Hydroclones are often used on low-weight, water-based muds to remove coarse drilled solids.
7.6.5.3.2.1 Desanders
Desanders consist of a battery of 6-inch or larger cones. Even though desanders can process large volumes of mud per single cone, the minimum size particles that can be removed are in the range of 40 microns (with 6-inch cones).
Figure 58 A desander.
7.6.5.3.2.2 Desilters
Figure 59 A desilter.
7.6.5.4 Mud Cleaner
The mud cleaner is a solids separation device that combines a desilter with a screen device. The mud cleaner removes solids using a two-stage process. First, the desilter processes the drilling fluid. Second, a high-energy, fine-mesh screen shaker processes the discharge from the desilter. This method of solids removal is recommended for muds containing significant amounts of weighting materials or having expensive fluid phases.
Note: When recovering weight material with a mud cleaner, be aware that any fine solids that go through the cleaner's screen are also retained in the mud. Over time, this process can lead to a fine-solids buildup.
Figure 60 Mud cleaners.
7.6.5.5 Degasser
The degasser is a device that removes the gas from the mud before it is recirculated down the hole. Gas-cut mud is mud containing gas. It should not be recirculated down the hole because it decreases the density of the mud, which could lead to a blowout.
There are two types of degassers:
• Mud-gas separators – for high-pressure gas and mudflows during kicks
• Vacuum degassers – for separating entrained gas 7.6.5.5.1 Vacuum degasser
The vacuum degasser gets mud from one of the mud pits. As the mud enters the top of the degasser and streams down an inclined plane that extends the whole length of the feed pipe, a vacuum in the vapor space causes the gas to leave the mud. The liberated gas is removed from the tank by a vacuum pump. The degassed mud returns to the pits ready to be recirculated.
Figure 61 Vacuum degasser.
7.6.5.5.2 Mud-gas separator
(See the section on Mud-gas separator under Well Control Equipment.)
7.7 Well Control Equipment
A kick is an influx of formation fluids into the well bore. This happens when the pressure in the formation (pore pressure) is higher than the mud density. A blowout is an uncontrolled kick. A blowout is very undesirable because:
• It endangers the lives of the crew.
• It destroys the rig.
• It may damage the environment.
7.7.1 Blowout Preventers
Blowout preventers or BOP’s, is a device attached to the casing head that in conjunction with other equipment and techniques, are used to close the well in and allow the crew to control a kick before it becomes a blowout. There are three basic types of blowout preventers on a drilling rig:
• Ram Preventers
• Rotating Head
Usually several blowout preventers are installed on top of the well in a stack, called a BOP stack. The annular preventer is always on top of the ram preventers. Several preventers are installed so that a kick, or impending blowout, can be controlled even if one of the preventers fails.
Figure 62 A BOP stack (from Phil Gavinda).
7.7.1.1 Annular Preventer
An annular preventer has a rubber sealing element that, when activated, closes tightly around the kelly, drill pipe or drill collars or the open hole (if there is no pipe in the hole). The seal is effected by compression of a reinforced elastomer packing element.
Figure 63 An annular preventer (from Hydril).
7.7.1.2 Ram Preventer
Ram preventers consist of rams (large steel valves) that have sealing elements.
There are generally three types of ram preventers:
• Pipe ram
• Blind ram
• Shear ram
Pipe rams close on the drill pipe and are not able to effect a seal on an open hole.
Blind rams are used to close an open hole effectively.
Shear rams are blind rams that have a cutting edge to cut the drill pipe completely, thus sealing the hole. They are used mainly in subsea systems.
Figure 64 Ram preventers (from Hydril).
7.7.1.3 Rotating Blowout Preventer
The rotating blowout preventer (RBOP) is also called a rotating head or a stripper head. They are continuously closed around the drill stem by means of a heavy-duty stripper rubber that rotates with the drill string. Packing provides a seal between the rotating and stationary elements.
Rotating heads are especially useful in drilling with air, gas, foam or any other drilling fluid whose hydrostatic pressure is less than the formation pressure.
Figure 65 Rotating blow out preventer.
7.7.1.4 Drilling Spool
Choke and kill lines may be connected to side outlets of blowout preventers. Using side outlets on BOP’s reduces the number of stack connections by eliminating the drilling spool.
spool the possible erosion to the less expensive piece of equipment is localized and it allows additional space between rams to facilitate stripping operations.
7.7.2 Accumulator
The blowout preventers are opened and closed by hydraulic fluid, which is stored under pressure in an accumulator. Several bottle- or ball-shaped containers are located in the operating unit, and the hydraulic fluid is stored in these containers. High-pressure lines connect the accumulator to the BOP stack. The accumulator is located at least 100 feet from the BOP stack.
Figure 66 Accumulators.
A master control panel is used to operate the preventers. A master control panel should be located at a safe distance from the wellbore. Another master control panel should be located on the rig floor or another place that is convenient for the driller to operate.
Figure 67 BOP master control panel on rig floor.
7.7.3 Choke Manifold
When a kick occurs, closing in the well using one or more of the blowout preventers is only the first step that must be taken to kill the well (bring the well under control). In order to kill the well, the kick must be circulated out and mud with the proper weight must be pumped and circulated.
The choke manifold is a series of valves, chokes, gauges and lines that is connected that controls the rate of flow from the well when the blowout preventers are closed.
When the well is shut in, the kick is circulated out through the choke line and manifold.
Figure 68 Choke manifold.
A choke is a valve. There are two types of chokes:
• Adjustable chokes
• Fixed chokes
An adjustable choke is operated hydraulically and has an opening that is capable of being restricted. The choke may vary infinitely in opening from fully closed to fully open. They are controlled from a remote panel on the rig floor.
A fixed choke has a flow restriction of permanent size.
7.7.4 Mud-Gas Separator
The mud-gas separator is essential for handling a gas kick. It separates the flammable gas from the mud so that the gas can be burned off (flared off) at a safe distance from the rig.
Most mud-gas separators are made of a vertical length of large diameter pipe. Interior baffles are used to slow the mud-gas stream. A gooseneck arrangement at the bottom allows the relatively gas-free mud to flow back to the shakers. The gas vent pipe at the top permits the gas to be flared without too much backpressure on the mud.
During a kick the well is shut-in and mud circulated through the choke manifold. The flow is diverted from the flow line or choke manifold to the mud-gas separator. The separator releases the gas that is then carried by the vent line at the top to a remote flare.
Figure 69 Mud-gas separator.
8 The Drilling Crew 8.1 Toolpusher
The toolpusher or rig superintendent is the man in charge of the rig and overall drilling operations. Generally, he is on call 24 hours a day and lives in the rig site. In many rigs, there are two toolpushers. A day toolpusher, who is the senior toolpusher, works during daytime and a night toolpusher who works the night shift.
8.2 Driller and Assistant Driller
The driller is in charge of drilling. He operates the drilling machinery. He is under the direct supervision of the toolpusher and is the overall supervisor of the floormen (or roughnecks). He and other members of the crew work 8- to 12-hour shifts, called tours (pronounced “towers”). Another driller and crew relieve them.
In most areas, the driller has an assistant called the assistant driller. The assistant driller is second in command in the rig floor and relieves the driller at intervals during their tour.
8.3 Derrickman
The derrickman in the absence of an assistant driller is second in command to the driller. He works on the monkeyboard when the pipe is being pulled out of or being run into the hole. The monkeyboard is a small platform up in the derrick. It is at a level even with upper end of a stand of drill pipe (about 90 feet of pipe equals one stand).
When the drilling is going on the derrickman is responsible for maintaining the mud or repairing the pumps and other circulating equipment.
In some rigs, a pumpman is in charge of maintaining the mud pumps. He is under the charge of the derrickman.
8.4 Motorman and Mechanic
The motorman is responsible for the engines, engine fuel, air compressors, water pump and other accessories.
The rig mechanic is an all around handyman when it comes to the mechanical components of the rig. In some rigs there are also assistant mechanics.
8.5 Electrician
The rig electrician maintains and repairs the electrical generating and distribution system on the rig.
8.6 Floorman
The floorman or roughneck is responsible for handling the drill pipe during connections or trips. A full drilling crew can have 2 to 4 roughnecks per tour. Floormen
8.7 Roustabouts
Roustabouts handle the equipment and supplies that are constantly being supplied to the rig. In some land rigs there are no roustabouts.
8.8 Crane Operator
The crane operator handles the cranes on offshore rigs to load and unload equipment, supplies and personnel.
9 Safety Provisions
To be published at a future date.
10 Drilling and Completing a Well 10.1 Introduction
Drilling a well determines whether the assumptions and/or estimates made are true.
Aside from proving or disproving the geological and geophysical estimates, the most important thing in drilling is to drill an in-gauge hole as quickly and safely as possible at the specified location and to the target depth.
The oil company employs a drilling contractor to drill a well. A number of service companies are also hired to perform services in aid of drilling. Their contracts are separate from that made with the drilling contractor. Some of these services are:
Drilling Fluid (Mud) Engineering
After a well is spudded, routine drilling consists of:
Drilling in increments the length of either one joint of pipe or a stand of three joints of pipe
Making connections
Tripping to change the bit when it is worn or is not suitable to drill the formation or to clean the hole
10.2 Routine Drilling Operations
10.2.1 ConnectionsWhen the kelly or top drive has been drilled all the way down (called the kelly down position), it is pulled up and a new joint or stand of drill pipe is added.